Millie iPad books are great because they are interactive and they include a combination of fictional illustrations and photo images with ‘real life’ video footage of Millie. Meet Millie is free but the other books usually cost about $1.00. I have purchased them all, however, Kathryn recently told me that Millie’s Crazy Dinosaur Adventure and Millie and the Lost Key are currently free. I am not sure how long they will be free for so I suggest that if you teach students who are aged 4 to 8, these books will be suitable for you so download them and see for yourself.
Source: Kathryn Bielby

If you do not have an iPad I have started to create a list of free online books on one of my Pinterest boards. You can access the board via this link or I have included a few links below. For the full list so far, visit my Pinterest page.
(1) Freekidsbooks.org (2) starfall.com (3) rif.org (4) readtomelev.com